Whirring noise 30+ mph
 

For the past few months i've been getting an intermittant whirring noise when the car gets up to speed. The shop hasent found anything in particular, and they have rotated the tires and done a "B" service on the car since this started. I mentioned it once but they dident seem to find anything. Could this be tires, or mabye a wheel bearing starting to fail? Abotu the same time i noticed my MPG has decreased a little bit as well, but i cant confirm it. any ideas?